The present invention is related to a fusion welding and can be used for a repair of various turbine engine components, more particular for repairing and manufacturing of turbine blades manufactured of equiaxed polycrystalline, single crystal and directionally solidified superalloys utilizing cladding and fusion welding processes.
In fusion welding, coalescence or joining between two or more articles takes place by melting of a base material with or without introduction of a filler material, followed by cooling and crystallization of a welding pool. Fusion welding can produce properties equal to those of the base material in wide range of temperatures and conditions. However, accommodation of solidification and residual stresses often results in cracking of difficult to weld Inconel 713, Inconel 738, Rene 77, Rene 80, Rene 142, CMSX-4, Rene N4, Rene N5 and other high gamma prime superalloys that have low ductility and prone to a liquation Heat Affected Zone (HAZ) cracking.
Brazing can produce crack free joints because it does not require melting of a base material to obtain coalescence. Brazing is carried out by melting and solidification of only brazing materials. However, the mechanical properties of brazed joints are usually below the mechanical properties of the base material by 50-75% at high temperature.
The poor mechanical properties of brazed joints produced by most nickel and cobalt brazing materials are related to a high content of boron in these materials and do not allow extensive dimensional restoration of turbine blades and structural repairs of other engine components.
Therefore, despite the propensity for cracking, welding is used more often than brazing for manufacturing and repair of different articles including turbine engine components. However, to avoid cracking during fusion welding turbine blades manufactured of materials having a low ductility are preheated prior to welding to a temperature exceeding 900° C. as per U.S. Pat. No. 5,897,801. Welding is accomplished by striking an arc in the preselected area so as to locally melt the parent material providing a filler metal having the same composition as the nickel-based superalloy of the article, and feeding the filler metal into the arc that results in melting and fusion of the latter with the parent material forming a weld deposit upon solidification.
A similar approach was utilized in the method disclosed in U.S. Pat. No. 6,659,332. The article is repaired by removing of damaged material that is present in the defective area, followed by preheating of the article to a temperature of 60-98% of the solidus temperature of the base material in a chamber containing a protective gas followed by welding.
In order to minimise welding stress in the blade due to the application of considerable thermal energy during fusion welding processes, blades are subjected to controlled heating prior to and controlling cooling after weld repair in accordance with the method described in CA 1207137.
Preheating of turbine blades increases the cost of a repair and does not guaranty crack free welds due to the low ductility of components manufactured of precipitation hardening superalloys.
Therefore, currently only preheating to temperatures exceeding 900° C. allows crack free welding on precipitation hardening equiaxed polycrystalline and directionally solidified high gamma-prime superalloys.
Therefore, one of major objectives of the present invention is the development of a new cost effective method of repairing and manufacturing of engine components by welding and cladding on polycrystalline, directionally solidified and single crystal superalloys at an ambient temperature.